The present invention relates to a device for measuring blood pressure.
More particularly, the present invention relates to a device for measuring the pressure of blood which is used in an extracorporeal blood treatment device in which the blood is taken from a patient in order to be treated then reintroduced into the body of the patient (especially for the purpose of carrying out dialysis) by means of an extracorporeal blood circuit comprising pipes and including at least one section for measuring the pressure of blood circulating in a pipe.
A known type of pressure measurement section forms a compartment which is delimited especially by a main wall and by a secondary wall facing it, the two walls being substantially rigid and parallel; the main wall comprises a hole which is sealed by a main closure element, the internal face of which is in contact with the blood and the external face of which is in contact with the ambient air, it being possible to elastically deform or displace the entire main closure element along a deformation or displacement axis which is substantially orthogonal to its general plane, under the effect of the blood pressure; a portion of the external face of the main closure element, in its rest state, is in direct or indirect contact with a load sensor which is able to measure the force applied axially to the internal face of the main closure element by the pressure of the blood, in order to calculate therefrom the value of this pressure.
Generally, this type of extracorporeal blood treatment device comprises a circuit part which is formed from a casing, or cassette, of the xe2x80x9cdisposablexe2x80x9d type, integrating pipes which are connected to the extracorporeal blood circuit.
The pressure measurement section may be an attached module which is mounted in a housing associated with the casing.
The casing is mounted on a support apparatus which comprises, for example, sensors, display means, pumping means, a control interface, an electronic control unit, etc.
In this type of extracorporeal blood treatment device, the blood pressure must be measured without contact between the measuring member and the blood.
Several systems for carrying out this pressure measurement are known.
In a first pressure measurement system, which is shown in FIG. 1, a pressure measurement section 10 in a pipe 12 comprises a measurement chamber 14 in which a membrane 16, or diaphragm, separates the blood flowing in the pipe 12 from the air contained in a compartment 18.
The membrane 16 can be deformed along a deformation axis Axe2x80x94A orthogonal to its general plane, such that it is axially displaced according to the blood pressure in the pipe 12.
The extreme deformation positions of the membrane 16 are shown in dotted lines.
The air compartment 18 is sealed shut when the pressure measurement section 10 is mounted on a support apparatus 20.
The support apparatus 20 comprises a sensor 22 which directly measures the pressure in the air compartment 18.
When the blood pressure changes, the membrane 16 is axially displaced to an equilibrium position in which the pressure is equal on each side of the membrane 16.
The pressure measured by the sensor 22 in the air compartment 18 is therefore equal to the blood pressure in the pipe 12.
By virtue of a suitable geometry, in particular by virtue of a suitable volume of the compartment 18 and a suitable surface for the membrane 16, this first pressure measurement system makes it possible to measure, on the one hand, so-called xe2x80x9cpositivexe2x80x9d blood pressures, that is blood pressures which are greater than a reference pressure, in this case atmospheric pressure, and, on the other hand, so-called xe2x80x9cnegativexe2x80x9d blood pressures, that is blood pressures which are less than the reference pressure.
This measurement system operates correctly provided there are no leaks in the air compartment 18, otherwise the membrane 16 is then displaced to its end stop and it no longer carries out the function of transmitting pressure.
The sealing of the air compartment 18 during mounting of the pressure measurement section 10 on the support apparatus 20 is a weak point of the measurement system.
In particular, the seal may be impaired while the measurement system is in use.
In a second pressure measurement system, which is shown in FIG. 2, the pressure measurement section 10 forms a compartment 24 containing the blood and a wall 26 of which includes a hole 28 which is sealed by a flexible membrane 30.
When the pressure measurement section 10 is mounted on the support apparatus 20, the external face of the central part of the flexible membrane 30 is in contact with a load transmitter 32 which is inserted between the membrane 30 and a load sensor 34.
The load sensor 34 makes it possible to measure the forces applied to the internal face of the membrane 30 due to the effect of the blood pressure in the compartment 24, when the blood pressure is greater than the ambient air pressure.
The blood pressure is calculated from the equation:                     P        =                              F            -                          F              0                                            S            a                                              (        1        )            
In this equation, F is the force measured by the load sensor 34, F0 is the force measured in the rest state, that is in the absence of a pressure gradient between the two sides (external and internal faces) of the membrane 30, and Sa is the area of the active surface of the membrane 30.
The area of the active surface Sa of the membrane 30 has a value between the total area of the internal face of the membrane 30 in contact with the blood and the area of contact between the membrane 30 and the load transmitter 32.
For very flexible membranes 30, the active surface Sa is substantially equivalent to the area of contact between the membrane 30 and the load transmitter 32.
This measurement system makes it possible to measure a positive pressure but it does not allow a negative pressure to be measured.
This is because, for negative pressures, the membrane 30 tends to come away from the load transmitter 32. The load sensor 34 can therefore no longer measure the forces which are applied to the membrane 30.
This system has therefore been adapted to measure negative pressures.
In order that the load sensor 34 can continue to measure the forces which are applied to the membrane 30, when the blood pressure is negative, the membrane 30 is secured in axial displacement to the load transmitter 32.
Thus, according to an improved embodiment of the second pressure measurement system, which is shown in FIG. 3, the membrane includes a metal disc 36 on its external face and the load transmitter 32 includes a magnet 38 at its axial end facing the membrane 30.
The magnetic attraction exerted by the magnet 38 on the metal disc 36 makes it possible to secure the membrane 30 in axial displacement to the load transmitter 32.
When the pressure is positive, the membrane 30 exerts an axial force which pushes against the load transmitter 32.
When the pressure is negative, the membrane 30 exerts an axial force which pulls on the load transmitter 32.
This device for securing the membrane 30 to the load transmitter 32 is expensive since it requires a special membrane 30 fitted with a metal disc 36 and a special load transmitter 32 fitted with a magnet 38.
The metal disc 36 must have a large area in order to allow effective magnetic coupling.
Furthermore, the membrane 30 experiences a significant jolt when the metal disc 36 xe2x80x9csticksxe2x80x9d to the magnet 38 of the load transmitter 32, which could impair its mechanical characteristics.
The invention aims to remedy these drawbacks.
For this purpose, the invention proposes a device for measuring the pressure of blood in a pipe of an extracorporeal blood circuit, comprising a pressure measurement section having a compartment which is delimited especially by a main wall and by a secondary wall facing it, the two walls being substantially rigid and parallel, the main wall having a hole which is sealed by a main closure element, the internal face of which is in contact with the blood and the external face of which is in contact with the ambient air, it being possible to elastically deform or displace the entire main closure element along a deformation or displacement axis, which is substantially orthogonal to its general plane, under the effect of the blood pressure, the main closure element being designed to engage with a load sensor so that a portion of the external face of the main closure element, in its rest state, is in direct or indirect contact with the load sensor which can measure the force applied axially to the internal face of the main closure element by the blood pressure, in order to calculate therefrom the value of this pressure, characterized in that the pressure measurement section comprises:
in its secondary wall, facing the hole of the main wall, a secondary hole which is sealed by a secondary closure element similar to the main closure element, the deformation or displacement axis of which is substantially coincident with that of the main closure element, the area of the internal face of the secondary closure element being greater than the area of the internal face of the main closure element, such that when the pressure of the blood is less than the pressure of the ambient air, the axial displacement of the secondary closure element towards the main closure element is greater than the axial displacement of the main closure element towards the secondary closure element;
and comprising, in the compartment, a transmission spacer which, when the pressure of the blood and the pressure of the ambient air are substantially equal, occupies a rest position in which it is in contact by a first axial end with the internal face of the main closure element and, by a second axial end, with the internal face of the secondary closure element;
such that when the blood pressure is less than the ambient air pressure, the spacer transmits the axial displacement, in the direction of the load sensor, from the secondary closure element to the main closure element, so that the load sensor can measure the resultant axial force in order to calculate therefrom the value of the blood pressure.
According to other characteristics of the invention:
the pressure measurement section comprises axial displacement guiding means for the transmission spacer;
the spacer has an axial rod provided, at least at one of its axial ends, with an axial support plate, the external face of which is adjacent and substantially parallel to the internal face of the associated closure element, when the spacer occupies its rest position;
the rod comprises a support plate at each one of its axial ends;
the area of the external face of each support plate is substantially equal to the area of the internal face of the associated closure element;
each of the closure elements and each of the support plates has substantially the shape of a disc;
the internal face of the main wall and the internal face of the secondary wall each comprise a rim, around the associated closure element, which extends axially towards the inside and which delimits a section of guide tube of a diameter substantially equal to the diameter of the associated support plate, for the purpose of axially guiding the transmission spacer;
the transmission spacer is attached to one of the closure elements;
at least one closure element is made in a single piece with the associated rigid wall, and the transmission spacer is made in a single piece with one closure element which is made in a single piece with the associated rigid wall;
the transmission spacer is made by moulding with a closure element which is itself made by moulding with the associated rigid wall;
the transmission spacer is secured in axial displacement to the secondary closure element;
the area of the secondary closure element is substantially twice the area of the main closure element;
the pressure measurement section comprises a sensor which identifies the direction of the axial displacement of the secondary closure element, so as to determine whether the axial force measured by the load sensor corresponds to a measurement of blood pressure which is above or below the pressure of the ambient air.